This application is based upon and claims priority of Japanese Patent Application No. 11-75025, filed Mar. 19, 1999, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical transmitter circuit which enables reliable monitoring of optical signals generated by a light emitting element, such as a semiconductor laser, with a comparatively inexpensive light receiving element, such as a photodiode. More particularly, the present invention relates to an optical transmitter circuit having a circuit to detect continuous xe2x80x9c1xe2x80x9d bits in the input data in order to perform update timing of a current control signal or deterioration evaluation of the light emitting element.
2. Description of the Related Art
Optical transmitter circuits are known. FIG. 13 illustrates an example of a conventional optical transmitter circuit, including a light emitting element 101 such as a semiconductor laser, a light receiving element 102 such as a photodiode to monitor light, a current-voltage converter (I/V) 103, an auto power control (APC) amplifier 104, a drive circuit 105, a sample hold circuit 106, and an analog switching circuit 107.
In operation of the conventional optical transmitter circuit shown in FIG. 13, the drive circuit 105 supplies drive current to the light emitting element 101 in accordance with input data (DATA). An optical signal is generated by the light emitting element 101 in accordance with the input data (DATA). The optical signal is detected by the monitoring light receiving element 102, converted into voltage by the current-voltage converting circuit 103, and input into the APC amplifier 104. The APC amplifier 104 compares the output signal of the current-voltage converting circuit 103 and a reference value, and inputs a signal corresponding to the comparative differential thereof into the sample hold circuit 106 via the analog switching circuit 107.
The analog switching circuit 107 receives the output signal of the APC amplifier 104 when the data (DATA) converted to optical signals is transmitted at level xe2x80x9c1xe2x80x9d, which output signal is input and held in the sample hold circuit 106. The held signal is input into the drive circuit 105 as a current control signal, and the drive current of the light emitting element 101 is controlled so that the optical output remains constant.
FIGS. 14A-14D are diagrams explaining the operation of the conventional example of the optical transmitter circuit shown in FIG. 13. More specifically, FIG. 14A illustrates the data (DATA) input to the drive circuit 105; FIG. 14B illustrates the optical output signal of the light emitting element 101; FIG. 14C illustrates the output signal of the current-voltage converting circuit 103; and FIG. 14D illustrates the drive current supplied to the light emitting element 101.
In the example shown in FIGS. 14A-14D, when the optical output signal of the light emitting element 101 corresponding to the data (DATA) that is input at time t1 decreases, as indicated by the OUTPUT DETERIORATION in FIG. 14B, the output signal of the current-voltage converting circuit 103 also decreases, as shown in FIG. 14C. The decreased output signal of the current voltage converting circuit 103 is held by the sample hold circuit 106 via the analog switching circuit 107, and a drive current is supplied to the light emitting element 101 from the drive circuit 105 as a current control signal corresponding to the data (DATA) that is input at the following time t2. More particularly, as shown in FIG. 14D, since the drive current corresponding to the data (DATA) that is input at time t2 is increased more than the drive current corresponding to the data (DATA) input at time t1, the optical output signal is controlled to a specified level, as indicated by the OUTPUT RECOVERY of FIG. 14B.
Furthermore, a circuit is known in which stabilization of the optical output is achieved by converting the output current of the light receiving element, which converts the optical output of the light emitting element, into voltage with a current-voltage converting circuit. When the converted optical output of the light emitting element reaches a specified level or above, the circuit determines the output to be a significant detection signal, holds it as a sample, and controls the drive current of the light emitting element in accordance with the held value. An example of this type of circuit is disclosed in Japanese Unexamined Laid-Open Patent Application Publication JP9-18054, wherein the input data is delayed and is considered to be a significant detection signal, and the output signal of the current-voltage converting circuit at this time is held as a sample.
When data (DATA) that is input is increased in speed, e.g., from a low speed of 50 Mbps to 150 Mbps, the light emitting element 101 has sufficient response speed since the light emitting element 101 is generally a semiconductor laser. However, a photodiode is generally used as the light receiving element 102. Because the photodiode has an increased surface area in order to increase the light receiving sensitivity, the capacitance CPD of the photodiode is generally, for example, 20 pF or above. Accordingly, when the current-voltage converting circuit 103 includes a resistance R, the band region fO is determined by the equation fO=xc2xdxcfx80RCPD, so it is extremely difficult to broaden the band. In other words, when a comparatively inexpensive photodiode is used as a light receiving element 102 to monitor the light emitting element 101, the response characteristics are not sufficient to detect an optical signal having a high speed of 150 Mbps or more.
FIGS. 15A-15D are diagrams illustrating problems occurring with the APC amplifier 104 operation in accordance with the conventional optical transmitter circuit. More specifically, FIG. 15A illustrates the data (DATA) input to the drive circuit 105; FIG. 15B illustrates the optical output signal of the light emitting element 101; FIG. 15C illustrates the output signal of the current-voltage converting circuit 103; and FIG. 15D illustrates the drive current supplied to the light emitting element 101. When the input shown in FIG. 15A occurs, the sample hold circuit 106 samples and holds the output signal of the APC amplifier 104 when the data (DATA) has been input. At this time, since the response speed of the light receiving element 102 is lower than the speed of the input data (DATA), the output signal of the current-voltage converting circuit 103 changes, as shown in FIG. 15C.
Moreover, the current control signal corresponding to the value that has been sampled and held is updated at the times t1, t2, t4, and t6, as indicated by the arrows showing the CURRENT UPDATE VALUE in FIG. 15, and the drive current supplied to the light emitting element 101 from the drive circuit 105 is updated. Accordingly, the output signal of the current-voltage converting circuit 103 resulting from the monitoring of the optical output produced by the light receiving element 102 in response to an initial input value of xe2x80x9c1xe2x80x9d becomes equal to or less than the set value indicated by the broken line at the time t1 in FIG. 15C. Because this output signal of the current-voltage converting circuit 103 is sampled and held, it is judged to be a lower optical output than a specified level, and the drive current supplied to the light emitting element 101 for the next continued data value of xe2x80x9c1xe2x80x9d increases, as shown in FIG. 15D. Accordingly, the optical output signal of the light emitting element 101 at this time becomes greater than the initial optical output, as shown in FIG. 15B.
In this state, at the following time t3, the drive current corresponding to the input data (DATA) xe2x80x9c1xe2x80x9d is supplied to the light emitting element 101. At this time, even if the optical output of the light emitting element 101 exceeds the specified level, the output signal of the current-voltage converting circuit 103 is lower than the set value at time t4 because of the response speed of the light receiving element 102, as shown in FIG. 15C. By sampling and holding the output signal of the current-voltage converting circuit 103, the drive current with respect to the input data (DATA) xe2x80x9c1xe2x80x9d at the next time t5 is further increased, as shown in FIG. 15D.
Monitoring the level of the output signals of the current-voltage converting circuit 103, and determining whether there is deterioration of the light emitting element 101 when this level falls below a specified value has been considered. However, since the response speed of the light receiving element 102 is insufficient as described above, there is a problem that a drop in the optical output level of the light emitting element 101 can be mistakenly assumed.
The above-described problem occurs when a photodiode which does not have a small capacitance is used as the light receiving element 102 in performing APC control. For this reason, the selection and use of a photodiode having a capacitance Cpd of 2 pF or less as the light receiving element 102 has been considered. However, this type of photodiode is extremely expensive. Photodiodes having high-speed response characteristics are also expensive. Thus, it has been extremely difficult to achieve cost reduction in the optical transmitter circuit which performs stabilization of the optical output and detection of deterioration in the optical signals with respect to high-speed data.
It is an object of the present invention to provide an optical transmitter which achieves stabilization of the optical output and thus enables a reliable detection of deterioration of a light emitting element, even when a photodiode which does not have a small capacitance is used as a light receiving element.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with an optical transmitter circuit comprising a light emitting element to convert input data to an optical output; a light receiving element to receive the optical output of the light emitting element and to output a current corresponding to the optical output; a current-voltage converting circuit to convert the output current of the light receiving element into a voltage and to output a voltage signal; an APC amplifier to compare the output voltage signal of the current-voltage converting circuit and a reference signal and to output a differential output signal; a hold circuit to hold the output signal of the APC amplifier and to form a current control signal; a drive circuit to receive the current control signal from the hold circuit and to supply drive current to the light emitting element in accordance with the current control signal; and a xe2x80x9c1xe2x80x9d continuous signal detecting circuit to detect a specified number of continuous of xe2x80x9c1xe2x80x9d bits in the input data and to perform an updating operation on the output signal of the amplifier held in the hold circuit.
In accordance with embodiments of the present invention, the hold circuit may comprise an analog switching circuit to receive the output signal of the amplifier and a detection signal of the xe2x80x9c1xe2x80x9d continuous signal detecting circuit, and to switch on in response to the xe2x80x9c1xe2x80x9d continuous signal detecting circuit detecting the specified number of continuous xe2x80x9c1xe2x80x9d bits; and a peak detecting circuit to detect and hold a peak value of the output signal of the AC amplifier that is input via the analog switching circuit.
The optical transmitter circuit may further comprise a peak detecting circuit to detect a peak value of the output signal of the current-voltage converting circuit and to input the peak value to the amplifier.
The optical transmitter circuit may further comprise a first peak detecting circuit to detect a peak value of the output signal of the current-voltage converting circuit and to output the peak value to the amplifier; and a second peak detecting circuit to detect a peak value of the input data and to output the peak value of the input data to the amplifier as a reference value.
In accordance with embodiments of the present invention, the hold circuit may comprise an up/down counter to count up or down corresponding to an output signal of the amplifier, in response to the xe2x80x9c1xe2x80x9d continuous signal detecting circuit detecting the specified number of continuous xe2x80x9c1xe2x80x9d bits; and a D/A converter to convert the contents of the count of the up/down counter to an analog signal and to input the contents of the count of the up/down counter to the drive circuit as an analog current control signal.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with an optical transmitter circuit, comprising a light emitting element to convert input data into an optical output; a light receiving element to receive the optical output of the light emitting element and to output a current corresponding to the optical output; a current-voltage converting circuit to convert the output current of the light receiving element into a voltage and to output a voltage signal; a light deterioration detecting comparator to detect deterioration in the light emitting element by comparing the output signal of the current-voltage converting circuit with a reference value and to output a deterioration detection signal; a xe2x80x9c1xe2x80x9d continuous signal detecting circuit to detect a specified number of continuous xe2x80x9c1xe2x80x9d bits in the input data; and a flip-flop to hold the deterioration detection signal from the light deterioration detection comparator in accordance with the detection signal from the xe2x80x9c1xe2x80x9d continuous signal detecting circuit.
In accordance with embodiments of the present invention, stabilization of the optical output of the light emitting element and detection of deterioration of the light emitting element can be performed. Furthermore, in accordance with embodiments of the present invention, stabilization of the hold value of the hold circuit or evaluation of light emitting element deterioration is performed by delaying the detection signal from the xe2x80x9c1xe2x80x9d continuous signal detecting circuit by a delay circuit.